H2CN/H2NC abundance ratio: a new potential temperature tracer for the interstellar medium

Abstract

ABSTRACT The H2NC radical is the high-energy metastable isomer of H2CN radical, which has been recently detected for the first time in the interstellar medium towards a handful of cold galactic sources, besides a warm galaxy in front of the PKS 1830−211 quasar. These detections have shown that the H2CN/H2NC isomeric ratio, likewise the HCN/HNC ratio, might increase with the kinetic temperature (Tkin), but the shortage of them in warm sources still prevents us from confirming this hypothesis and shedding light on their chemistry. In this work, we present the first detection of H2CN and H2NC towards a warm galactic source, the G+0.693−0.027 molecular cloud (with Tkin > 70 K), using IRAM 30-m telescope observations. We have detected multiple hyperfine components of the $N_{K_\text{a}K_\text{c}} =$ 101–000 and 202–101 transitions. We derived molecular abundances with respect to H2 of (6.8 ± 1.3) × 10−11 for H2CN and of (3.1 ± 0.7) × 10−11 for H2NC, and an H2CN/H2NC abundance ratio of 2.2 ± 0.5. These detections confirm that the H2CN/H2NC ratio is ≳2 for sources with Tkin > 70 K, larger than the ∼1 ratios previously found in colder cores (Tkin ∼ 10 K). This isomeric ratio dependence on temperature cannot be fully explained with the currently proposed gas-phase formation and destruction pathways. Grain surface reactions, including the H2NC → H2CN isomerization, deserve consideration to explain the higher isomeric ratios and H2CN abundances observed in warm sources, where the molecules can be desorbed into the gas phase through thermal and/or shock-induced mechanisms.